Adhesive labels and tapes are well known. In a typical label construction, one or more layers of adhesive are coated on or otherwise applied to a release liner, and then laminated to a facestock, such as paper, polymeric film, or other ink-receptive, flexible material. In a typical tape construction, a polymeric film or woven paper is coated with an adhesive on one surface, which is then wound up upon itself. A release liner is not generally required. Labels are usually die-cut and matrix-stripped before use. In contrast, tapes usually do not require die-cutting and matrix-stripping, and generally need not be ink-receptive.
The adhesives used in both tapes and labels include pressure-sensitive adhesives (PSAs). Both rubber-based and acrylic-based PSAs are known; most contain one or more tackifiers that improve overall adhesion to various substrates. PSAs can be applied to a release liner or facestock from an organic solvent, from an aqueous dispersion, or as a hot melt. Hot melt PSAs (HMPSAs) are highly desirable, as the absence of solvent or water lowers the energy required to form the adhesive layer and reduces the environmental problems associated with solvent-borne adhesives. However, most HMPSAs are somewhat volatile and contain volatile organic compounds (VOCs).
A typical rubber-based HMPSA composition contains one or more natural or synthetic elastomers, tackified with a petroleum resin and/or other ingredients, such as plasticizers, that improve the tack of the adhesive. Certain elastomeric block copolymers, such as Shell's Kraton.RTM. block copolymers are widely used as the polymeric components in HMPSAs.
Natural and synthetic elastomers containing polybutadiene segments and polyisoprene segments are not generally miscible with each other. It is known in the art, however, to mix immiscible elastomeric polymers based on polybutadiene and polyisoprene. Homopolymers are generally more difficult to employ then styrenic block copolymers. With styrenic block copolymers, while the midblocks are immiscible, the end blocks of polystyrene form one common domain, thus stabilizing the mixture and eliminating some or all of the problematic effects of incompatibility, such as separation of the elastomeric phases over time.
One problem with PSAs based on tackified elastomeric blends is diffusion and migration of tackifiers and other species into the facestock. As a result, the facestock may become stained over time, and the construction may lose some adhesion. Although an intermediate barrier layer can be positioned between the facestock and the adhesive, such an approach complicates the manufacturing process and increases the cost of the construction.
During label manufacture, a laminate of a facestock, PSA layer and a release liner is passed through apparatus that converts the laminate into commercially useful labels and label stock. The processes involved in the converting operation include printing, die-cutting and matrix-stripping to leave labels on a release liner, butt-cutting of labels to the release liner, marginal hole punching, perforating, fan folding, guillotining and the like. Die-cutting involves cutting of the laminate to the surface of the release liner. Hole punching, perforating and guillotining involve cutting cleanly through the label laminate.
The cost of converting a laminate into a finished product is a function of the speed and efficiency at which the various processing operations occur. While the nature of all layers of the laminate can impact the ease and cost of convertibility, the adhesive layer typically has been the greatest limiting factor in ease of convertibility. This is due to the viscoelastic nature of the adhesive, which hampers precise and clean penetration of a die in die-cutting operations and promotes adherence to die-cutting blades and the like in cutting operations. Stringiness of the adhesive also impacts matrix-stripping operations, which follow die-cutting operations.
Achieving good convertibility does not, by necessity, coincide with achieving excellent adhesive performance. Adhesives must be formulated to fit specific performance requirements, including sufficient shear, peel adhesion, tack or quick stick, at various temperatures. A good, general purpose adhesive may exhibit poor convertibility simply because the adhesive is difficult to cleanly cut. Such an adhesive may stick to a die or cutting blade during converting operations. In general, the softer the elastomer(s) and resin(s) used in the composition, the more readily the adhesive will adhere to a surface. However, if the composition is too soft and tacky, its cohesive strength (shear strength) will be low, and the composition may split or shear easily under stress, or ooze under pressure. An ideal HMPSA would have both high tack and high cohesive strength and, in addition, would have good flow characteristics if it is to be used in the bulk state, so that it can be coated or otherwise applied to a facestock or, coated on a release liner and laminated to a facestock.
A variety of petroleum resins and other compounds are used as PSA tackifiers. For Kraton.RTM. type block copolymers, it is known to use tackifiers prepared according to the teachings of U.S. Pat. Nos. 3,577,398 (Pace) and 3,692,756 (St. Cyr), wherein the tackifiers are obtained by polymerization of a stream of aliphatic petroleum derivatives in the form of dienes and monoolefins containing five or six carbon atoms. The resulting petroleum resins typically are normally liquid at room temperature or normally solid at room temperature, and generally have low or high softening points, respectively. Intermediate softening point resins, however, have heretofore either not been readily available or not utilized in the tackification of block copolymer elastomers.
HMPSAs formulated with the resins described in the aforementioned two patents tend to exhibit poor adhesion to corrugated cardboard substrates. In addition, unless one or more plasticizing components is included in the formulation, HMPSAs tackified with such resins generally cannot be used across a wide range of application temperatures. What is needed is an HMPSA composition having low volatility that adheres well to a variety of substrates over a broad temperature range, and that does not exhibit substantial bleed or facestock staining.